1. Field of the Invention
The present invention pertains to optics, and in particular, to beam splitters used in microlithography.
2. Related Art
Photolithography (also called microlithography) is a semiconductor fabrication technology. Photolithography uses ultraviolet or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as, diodes, transistors, and integrated circuits, can be fabricated using photolithographic techniques. Exposure systems or tools are used to carry out photolithographic techniques, such as etching, in semiconductor fabrication. An exposure system can include a light source, reticle, optical reduction system, and a wafer alignment stage. An image of a semiconductor pattern is printed or fabricated on the reticle (also called a mask). A light source illuminates the reticle to generate an image of the particular reticle pattern. An optical reduction system is used to pass a high-quality image of the reticle pattern to a wafer. See, Nonogaki et al., Microlithography Fundamentals in Semiconductor Devices and Fabrication Technology, Marcel Dekker, Inc., New York, N.Y. (1998), incorporated in its entirety herein by reference.
Integrated circuit designs are becoming increasingly complex. The number of components and integration density of components in layouts is increasing. Demand for an ever-decreasing minimum feature size is high. The minimum feature size (also called line width) refers to the smallest dimension of a semiconductor feature that can be fabricated within acceptable tolerances. As a result, it is increasingly important that photolithographic systems and techniques provide a higher resolution.
One approach to improve resolution is to shorten the wavelength of light used in fabrication. Increasing the numerical aperture (NA) of the optical reduction system also improves resolution. Indeed, commercial exposure systems have been developed with decreasing wavelengths of light and increasing NA.
Catadioptric optical reduction systems include a mirror that reflects the imaging light after it passes through the reticle onto a wafer. A beam splitter cube is used in the optical path of the system. A conventional beam splitter cube, however, transmits about 50% of input light and reflects about 50% of the input light. Thus, depending upon the particular configuration of optical paths, significant light loss can occur at the beam splitter.
In UV photolithography, however, it is important to maintain a high light transmissivity through an optical reduction system with little or no loss. Exposure time and the overall semiconductor fabrication time depends upon the intensity or magnitude of light output onto the wafer. To reduce light loss at the beam splitter, a polarizing beam splitter and quarter-wave plates are used.
Generally, polarizing beam splitters are designed for maximum optical throughput, but without a particular attention to the apodization they impose on the pupil of the projection optics. In optical systems having low numerical apertures (i.e., on numerical apertures corresponding to a lower range of operating angles at the beam splitter coating), this is not a significant problem, since the natural bandwidth of the coating is typically large enough to cover the requirements. However, at higher numerical apertures, the coating designs become more complex, and result in an increase in undesirable performance fluctuations over the angular range of operation.
Accordingly, what is needed is a beamsplitter with a relatively flat apodization function over a wide angular range that is usable in UV photolithography.